1. Field of the Invention
This invention relates generally to a system for pumping fluid from a well. More specifically, this invention relates to a system in which a subsurface pump is driven by axial motion of the tubing through which the fluid is produced.
2. Description of Related Art
Two types of tubing are popular in oil wells: threaded tubing and coiled tubing. Threaded tubing consists of fixed lengths of pipe with threaded ends that allow the threaded tubing to be coupled together to form the tubing string. On the other hand, coiled tubing is a long, continuous pipe which is unwound from a spool as it is fed into the well. While each tubing type has its advantages, coiled tubing is generally regarded as more economical. Also, coiled tubing can be used in smaller diameters than threaded tubing.
A significant portion of the oil and gas produced every year is extracted from shallow, low-volume "stripper" wells. Such wells may employ a pump system configuration such as that shown in FIG. 1A. In pump system 400, a wellbore casing 401 extends downward from the surface to various production formations 405. Casing 401 has perforations 410 in the region of the various production formations 405 that allow fluids from these formations to enter the wellbore. A tubing string 402 is situated within casing 401 and also extends downward from a well-head (not shown) to approximately the level of the production formations 405. Located at the terminal end of tubing string 402 is a plunger 404 which resides in a pump chamber 406. The plunger 404 is connected to a solid "sucker rod" string 403, which is in turn connected to a beam pumping unit 408 at the surface. Beam pumping unit 408 raises and lowers the sucker rod 403 to move the plunger 404 in relation to the pump chamber 406. The plunger motion, in conjunction with the operation of check valves 407, causes the transfer of fluids from below the plunger 404 to the annulus between the sucker rod 403 and tubing 402. Repeated raising and lowering of plunger 404 eventually transfers sufficient fluid to fill tubing 402 and to thereafter force fluid to flow from well at the surface, creating a producing well. FIG. 1B shows an enlarged cross-sectional view of the subsurface pump.
The hole that is initially drilled for a well must have a sufficiently large diameter to accommodate at least casing 401, tubing 402, and sucker rod 403, along with any additional clearances required by their couplings. Often additional, larger-diameter casings may also be provided near the surface. Since larger holes require larger drilling equipment, longer drilling times, and higher pipe costs, it is desirable to minimize hole diameters as much as possible without reducing the production rate of the well.